In recent years, various kinds of liquid crystal displays have been developed and supplied. In these kinds, reflection type liquid crystal displays make a display by reflecting light in the circumference (external light). Therefore, reflection type liquid crystal displays consume less electric power to make a display compared with transmitting type liquid crystal displays, and because of this advantage, liquid crystal displays of this type are used as displays of mobile equipment such as mobile telephones. Further, studies and developments of displays which consume less electric power have been done, and reflective type liquid crystal displays with a memory effect have been suggested.
In page 897 of SID International Symposium Digest of Technical Paper Vol. 29, the operation mode of a reflective type liquid crystal display with a memory effect is disclosed. In this operation mode, the alignment state of chiral nematic liquid crystal is switched between a planar state (selective reflection state) and a focal-conic state (transmitting state), and thereby, the liquid crystal display makes a display. The planar state and the focal-conic state are stable, and once the liquid crystal is set in either of the state, the liquid crystal keeps in the state permanently as long as no external force is applied. In other words, once an image is written on the liquid crystal display, the image keeps being displayed even after the power source is turned off. Thus, this liquid crystal display can be used as a reflective type liquid crystal display with a memory effect.
The reflective type liquid crystal display disclosed in the reference above is of a structure wherein chiral nematic liquid crystal is filled between a pair of substrates with respective electrodes thereon. Because of the electrodes, an electric field acts in a vertical direction to the substrates, and by controlling the strength and/or the application time of the electric field, the liquid crystal is set to a specified state (a planar state or a focal-conic state).
When a voltage which is higher than a threshold voltage to untwist the liquid crystal is applied to the liquid crystal for a sufficiently long time, the liquid crystal entirely comes to a homeotropic state (wherein the longer axes of liquid crystal molecules are perpendicular to the substrates). This state is not stable, and when the electric field is turned off thereafter, the liquid crystal is twisted. When the electric field is erased suddenly from the homeotropic state, the liquid crystal comes to a planar state. When the electric field is erased gradually, the liquid crystal comes to a focal-conic state.
When a pulse voltage which is higher than the threshold voltage to untwist the liquid crystal (a pulse voltage with such a width as to cause part of the liquid crystal to come to a homeotropic state) is applied to the liquid crystal in a focal-conic state, the liquid crystal comes to a homeotropic state and thereafter comes to a planar state on completion of the pulse voltage application. By controlling the width and the height of the pulse voltage, the rate of the liquid crystal which comes to a planar state can be controlled, and thereby, display of intermediate colors is possible.
The liquid crystal display which uses chiral nematic liquid crystal, however, has the disadvantage of requiring a high voltage to write an image thereon. Specifically, when a new image is to be written on the liquid crystal which is already set in a focal-conic state or in a planar state, that is, on the liquid crystal on which an image is already written, the liquid crystal must be untwisted and set in a homeotropic state once, which requires a high driving voltage. In a homeotropic state, visible light is absorbed in a light absorbing layer which is provided on the back side of the liquid crystal display, and the entire screen becomes dark momentarily and is difficult to see.
An object of the present invention is to provide a liquid crystal display which is driven by a low voltage and which has a screen which is not difficult to see during writing of an image thereon.